1. Field of the Invention
The present invention relates to an optical demultiplexing (DEMUX) apparatus for extracting a signal light of a reference rate, which configures a signal light multiplexed with electrical time-division multiplexing or optical time-division multiplexing (ETDM or OTDM).
2. Description of the Related Art
ETDM/OTDM are the promising multiplexing technologies for speeding up an optical network, and for increasing a capacity in terms of an implementation of an ultra-fast optical network in the future. Wavelength division multiplexing (WDM) is the most suitable multiplexing technology for a long-haul transmission. However, since an optical carrier of wavelengths the number of which is equal to the number of channels is required, configuration of an optical network becomes complex. Although the bit rate of an ETDM/OTDM signal becomes high at times equal to the number of channels, it can be transmitted over single optical carrier of a single wavelength. Its transmission distance is limited in correspondence with an increase in the bit rate. However, if such a signal is used for a short-distance transmission or for signal processing within an optical node of an optical network, a configuration can be significantly reduced due to its single wavelength in comparison with the case where WDM is used. Additionally, in the processing executed within an optical node, optical switching in a time regime can be used. Therefore, a switching speed can be improved on the order of 106 to 107 times in comparison with the case where a wavelength switch is used.
One of the major problems when an ETDM/OTDM signal is used is demultiplexing.
For example, an OTDM signal of 160 Gbit/s is multiplexed with a short pulse having a full width at falf maximum (FWHM) equal to or smaller than about 2 ps or so while timing is shifted at an interval of 6.25 ps for each channel. To demultiplex a signal of 10 or 40 Gbits/s signal, which is a reference rate, from such an OTDM signal of 160 Gbits/s, an optical switch which has a high-speed operation characteristic of order of picoseconds or smaller, whose extraction time interval (Window) is sufficiently shorter than 6.25 ps, and which can sufficiently eliminate adjacent channels is required. Such high-speed optical demultiplexing is difficult to be made electrically because of restrictions on the operation speed of electronics. An optical gate of a Mach-Zehnder interferometer type, an optical switch using four-wave mixing or an optical chirp, a configuration where Ea modulators are connected in series, etc. have been proposed and studied so far.
With an optical demultiplexing circuit using an optical AND circuit adopting a nonlinear effect, a channel is demultiplexed by a control light pulse as indicated by Japanese Patent Application Publication No. 8-195732. Even with this circuit, an optical pulse after being demultiplexed includes much cross talk depending on the value of a half value width of the control light pulse.
Additionally, Japanese Patent Application Publication No. 2001-27769 recites a configuration using an optical switch. Also in this case, an optical pulse after being demultiplexed includes cross talk depending on the value of a half value width of a control light pulse.
As described above, with conventional various optical demultiplexing apparatuses, a control light composed of optical pulses having a narrow pulse width is interacted with a signal light, and only a necessary optical pulse is extracted. However, the S/N ratio of the extracted optical pulse depends on the degree of the narrowness of the width of the optical pulse of the control light. Namely, unless the FWHM of the optical pulse of the control light is sufficiently small, some optical pulses adjacent to a target optical pulse are also extracted together when the particular optical pulse is extracted from a signal light having a very high bit rate in the future. This causes noise to the target optical pulse. As the bit rate of a signal light increases, the pulse width of the optical pulse of the signal light decreases. Therefore, also the FWHM of the optical pulse of control light must become smaller in correspondence with this phenomenon. Actually, however, it is difficult to generate an optical pulse of control light having such a small pulse width, and accordingly, an apparatus for generating such an optical pulse becomes complex and expensive.